The processes used to treat waste from different industries are based on the example of large facilities including furnaces in which smelting, vitrifying or volatilizing the waste to be recovered, inerted or eliminated are carried out, which facilities are generally costly and fairly energy inefficient, whereby the waste recovery treatment becomes a very expensive process.
When molten metal is used, the treatment of the waste having a lower density than the density of molten metal is complicated due to the fact that such waste is not introduced therein as a consequence of there being a slag layer on the surface thereof, the lower density of which prevents contact of the waste with the molten metal.
Likewise the waste normally incorporates materials from different mixed classes, such as plastics, metals and others which, upon smelting, can cause contaminating gases or fumes that are difficult to treat. This implies having to carry out prior waste separation or selection treatments.
One type of waste to be treated would be steelmaking dust for example, containing large quantities of Zn, Pb or Cd and which are generated in refining and smelting operations in the steel production processes, collected in fume purification systems, and which form dangerous waste mainly due to the presence of Pb or Cd.
Currently, steelmaking dust is treated in different ways:                Dumped into dumping sites for hazardous products. This is the least used option since it is costly and there are other better alternatives.        Inerting by means of mixing agglomerating additives capable of generating an encapsulation that allows passing the leaching test established by law.        Recovering the valorizable metals by heat treatment and separating and purifying the gases containing volatilizable metals.        
There is a considerable number of methods designed specifically for treating steelmaking dust that intend to reach the objective of recovering the metals contained in said waste by means of applying very diverse techniques. Thus, for example, U.S. Pat. Nos. 6,322,745 , 5,942,023 and 6,494,933 describe treatment methods in which both the technique used and the result obtained differ substantially according to the applied process. The mentioned patents comprise the state of the art currently in force, since it includes treating steelmaking dust by the three most used methods worldwide.
In these processes, reducing iron oxide contained in steelmaking dust is achieved in all the cases by means of feeding in a carbonaceous material as a reducer element. The differences lie in the way and moment in which the materials are introduced into the furnace, the geometry of the furnace and the heating system used.
Said differences can be seen particularly in the operating limitations that the different processes have. Heating systems can be mentioned as an example. The patents mentioned use, respectively in the indicated order, carbon combustion by oxygen injection, DC plasma arc and carbon combustion by blowing air.
With oxygen injection and plasma, high process temperatures are obtained causing important deterioration problems in the furnace refractories or reactors. The urgent necessity for panels and refrigeration systems directly impacts reducing energy efficiency.
The temperatures are lower with air blowing, but a variable part of the iron contained in the steelmaking dust is not reduced, and the material obtained must be further reprocessed in an arc furnace because the smelting temperature of the metal which would allow directly extracting the iron is not reached.
Another type of hard-to-treat waste would be aluminum chips having a very small density compared to molten metal, making the chips difficult to smelt and likewise problematic in that they easily rust in contact with air. A device for treating them is described in U.S. Pat. No. 6,036,745. This case describes a large furnace which communicates with two interconnected tanks to which the molten metal is conveyed from the furnace with the aid of a pump or pump unit. In the first tank, on one hand the solid metal particles are introduced from the outside and on the other hand the molten metal is introduced from the furnace, the mixture thus formed is then conveyed to a second tank from which the impurities, scum and slag from the bath are conveyed towards the outside, returning the molten metal to the furnace.
This furnace has a small outlet on one side to cause, with the aid of a pump, metal to flow towards the first tank in which a diverter is located, creating a spiral circulation facilitating the immersion of the waste particles in the flow of metal, for then passing this flow of metal through a conduit located at the bottom of the mentioned first tank, to a second tank in which the scum, slag and impurities are released, as has been previously mentioned, and then the flow of metal is introduced into the furnace.
Therefore, aluminum slag or chip treatment is carried out by immersion in the melt contained in the first tank by means of generating a downward spiral flow dragging the chips from the surface of the molten metal to the interior thereof.
This is a large furnace, is heated by resistances needing great potential and it requires a large length, basically making it cost-effective in large waste treatment facilities.
The installation has a complicated geometry due to incorporating on one hand the furnace and on the other hand the interconnected tanks and the communication conduits, increasing complexity of maintaining the facility, turning it off, turning it on and draining it.
Likewise it must be pointed out that in the installation object of this patent, submerged pumps are used to move the molten metal with the complexity that the maintenance thereof implies.
This installation is suitable for treating aluminum chips, but is dangerous due to the quick immersion of aluminum in the molten metal melt, since it can cause explosions if the aluminum chips contain water or moisture, given that this water is converted into hydrogen, which can violently explode.
Other installations are described in U.S. Pat. Nos. 6,217,823, 4,598,899 and 5,143,357 , all of which focus on aluminum chip treatment